Old Stellar Populations in Galaxy Clusters

1. Old Stellar Populations in Galaxy Clusters Scott Trager (Kapteyn Institute, Groningen) with Sandy Faber (UCO/Lick) Alan Dressler (Carnegie Observatories) Stellar Populations, MPA

2. Introduction: Why Cluster Galaxies? • Why early-type galaxies? • Spheroidal galaxies (E’s, S0’s, and bulges) contain 50% (Schechter & Dressler 1987) to 75% (Fukugita et al. 1998) of stellar mass in Universe • Why clusters? • Easy place to find them—80% of ellipticals live in clusters, at least 50% of S0s • Aren’t cluster early-types boring, forming all their stars at z>3? • Maybe…maybe not… Stellar Populations, MPA

3. Observed clusters • Local Universe: rich and nearby… • Nearby: • Virgo (Gonzàlez 1993; Caldwell et al. 2003; Yamada et al.) • Fornax (Kuntschner 2000) • Hydra (Trager, in prep.) • Rich: • Coma (cf. Jørgensen 1999; Mehlert et al. 2000; Terlevich et al. 2001; Poggianti et al. 2001; Kuntschner et al. 2001; Moore et al. 2002; Sánchez-Blázquez et al.; Trager et al.) • Distant Universe: rich clusters (so far…) • Abell 851 (=CL0939+4713), z=0.41 • GHO1322+3114 (z=0.69), GHO1322+3027 (z=0.75) Stellar Populations, MPA

4. Coma: our study • Centered around NGC 4874, one of two cDs in center of Coma • 30s “white light” picture from LRIS • Early-types with Mb<-16 Stellar Populations, MPA

5. <Fe> Mgb Hβ Hγ Fe4383 Hδ C24668 Really good spectra! • Coma early-type galaxy spectra taken with Keck/LRIS • 2 hours through clouds, 0”8 seeing • Typical S/N>150/Å Stellar Populations, MPA

6. OK, the boring stuff • Calibration to Lick/IDS system • Relative flux correction • Smooth to Lick/IDS resolution • Measure line strengths and errors (à la Gonzàlez 1993) through correct bandpasses • Fixed by cross-correlation of each index with HR 6018, the template star for the Lick/IDS system • Calibrate stellar indices onto Lick/IDS stellar system • Small additive corrections needed for some indices • Velocity dispersion corrections to get galaxies onto stellar system (à la Trager et al. 1998) • Et voilà! We’re on the Lick/IDS system… Stellar Populations, MPA

7. Stellar Populations of Coma Early-types NGC4867 Red: field & Virgo Es from Gonzàlez (1993); Green: Fornax early-types from Kuntschner (2000) Models from Thomas, Maraston & Bender (2003) NGC4874 • Early-types in Coma are nearly coeval • but not exactly: Factor of two spread in age, even in the massive galaxies! • NGC4874 is 12 Gyr old, NGC4867 is 5 Gyr old • note that error bars are mostly smaller than points • But much more than even early-types in the field (red) or even Fornax (green)! Stellar Populations, MPA

8. The Z-plane • Trager et al. (2000b) found that local early-type galaxies project onto a plane in the four-space of log t, [Z/H], [E/Fe], and logσ • Controlling parameters can be identified with log t and log σ • Coma early-types obey this plane as well • Except D158 and GMP3565, which seem to be too metal-poor for their masses Models from Worthey (1994) + Trager et al (2000a) Stellar Populations, MPA

9. Correlations with σ • [E/Fe]-σ relation, so strong in field galaxies (Trager et al. 2000b), clearly evident in Coma galaxies • But note slight offset and GMP3565 • Are there two [E/Fe]-σ relations? • No age-σ relation, but obvious factor of two spread in age at all masses • Late episodes of star formation are stochastic, independent of galaxy mass • Mass-metallicity relation exists in Coma (if not the field) • Note again that D158 and GMP3565 too metal-poor for their masses Stellar Populations, MPA

10. Stellar population parameters from Hδ and Hγ: effects of [E/Fe] • No corrections for effect of [E/Fe] on Hδ and Hγ currently published • Excellent Coma spectra allow us to make empirical corrections • Predict line strengths from models of Hβ, Mgb, <Fe> • Strong effect! • cf. Korn, Maraston et al. Stellar Populations, MPA

11. The effect of [E/Fe] and [C/Fe] on C24668 and Fe4383 • Again, use Hβ, Mgb, <Fe> to predict C24668 and Fe4383 • Highly useful for intermediate-z galaxies • Doesn’t quite work • Instead, allow [C/E] to vary independently of other “E” elements • Works very well, only requires typical [C/E]=0.04±0.02(!) • cf. Sánchez-Blázquez et al. (2003) • Note: also solves offset in [E/Fe]-σ relation Stellar Populations, MPA

12. Putting it all together… • Stellar parameters from Hδ or Hγ, C24668, Fe4383 vs. Hβ, Mgb, <Fe> • Make empirical corrections to higher-order Balmers • Pretty good fit for t • [Z/H] slightly too high • [E/Fe] is off but linearly related to original values Stellar Populations, MPA

13. Distant galaxies: The New Frontier Trager, Dressler & Faber, in prep. • Stellar populations of early-type galaxies in A851 (CL0939+4713) at z=0.4 • Can now determine SSP ages and abundances of galaxies out to z=0.8 (at least), using corrections determined from Coma galaxies Stellar Populations, MPA

14. What’s next? Mn Sr Cr Mn • PCA analysis of Coma spectra • Can begin to study interesting elements besides Mg, Fe, CN, Ca, Na • in the blue: Mn, Sr (see Rose 1985), Cr, (Zn?) • in the red: Ba (others?) • but still not O! Trager, in prep Stellar Populations, MPA

15. Conclusions • Coma early-types are not coeval, even in cluster center • Not just faint galaxies—even massive ellipticals show age spread of 2x • But much more coeval than local field or even Fornax S0s • Galaxies follow Z-plane, but low-mass galaxies are off • Separate relation for dEs? • No mass-age relation  star formation events are stochastic Stellar Populations, MPA

16. Conclusions (cont.) • [C/E]>0 ? • Possible, but not highly enhanced • <[C/E]>=0.04±0.02 for massive Coma early-types • Maybe C stars? • Can determine stellar populations from higher-order Balmers (Hγ, Hδ) and C24668, Fe4383 with confidence • Allows us to study higher-redshift galaxies! • End of days for line strengths? • Much better spectra and much better libraries  new tools for stellar population analysis (cf. Vazdekis 1999) • But still need to correct for [X/Fe]... Stellar Populations, MPA

17. Special thanks to Guy Worthey, Dan Kelson, Daniel Thomas, Claudia Maraston, Harald Kuntschner, and Bianca Poggianti Stellar Populations, MPA

18. Continuing Annoyances • O abundances • need accurate O abundances for precise age dating of old stellar populations (Worthey), but no good way of measuring in optical • what about IR OH lines? • Stellar libraries • still dependent on Lick/IDS stellar library, but errors are large compared with current galaxy and GC data • do not (yet?) include metal-rich, α-enhanced stars • BHB stars in metal-rich populations • very likely to be small contribution in gE’s (Rose 1985; Maraston & Thomas 2000) • BHBs have been seen in metal-rich bulge GCs (Rich et al. 1997) • Blue stragglers • unlikely to be significant contributors to Balmer-line strengths (Rose 1985) • What is the temperature of the RGB tip? Stellar Populations, MPA

19. Metallicity distributions • Multiple ages cause problems... • ...but metallicity distributions aren’t so bad • for M32, assuming 8.5 Gyr population and Grillmair et al. (1996) MDF, find • Hβ=2.0Å, Mgb=2.9Å, <Fe>=2.3Å (model) • Hβ=1.9Å, Mgb=3.0Å, <Fe>=2.4Å (observed) • gives [Z/H]SSP=-0.32 • compare to light-weighted peak of [Z/H]=-0.25 from CMD • but only works in absence of metal-poor tail [Fe/H]=-1.2 [Fe/H]=0 SSP peak light-weighted peak Stellar Populations, MPA